A patch antenna is a planar antenna constructed such that a dielectric substrate forming a patch electrode on its upper face is arranged on a ground face, and a predetermined high frequency electric current is supplied to this patch electrode through a power supply terminal, etc. For example, the patch antenna is used as various antennas such as a base station antenna, etc. in a communication system of a portable telephone, etc.
FIGS. 11A and 11B show the structure of a patch antenna 20 using the dielectric substrate. FIG. 11A is a plan view of the patch antenna 20. FIG. 11B is a cross-sectional view seen from line B-B′ of the patch antenna 20 shown in FIG. 11A. As shown in FIG. 11B, in the patch antenna 20, an antenna radiating element 22 is formed from copper foil in a pattern on one face of the dielectric substrate 21. Further, a GND 23 is formed on the side opposite the antenna radiating element 22 through the dielectric substrate 21. In the patch antenna 20 shown in FIGS. 11A and 11B, the input impedance of an edge of the antenna radiating element 22 is 200 ohms or more. Therefore, when a signal of 50 ohms is input from a communication device, etc. to the edge of the antenna radiating element 22 as it is, loss of power due to reflection is increased.
Therefore, a power supply method such as offset power supply is used. In the offset power supply, as shown in FIG. 11A, a signal is not supplied from the edge of the antenna radiating element 22, but is supplied from an internal area A of the antenna radiating element 22 lower in impedance than the edge. Thus, the impedance is matched and the loss of power due to reflection is reduced. Further, in the patch antenna 20, a central conductor 27 of a coaxial line path is connected to a power supply point 24 of the antenna radiating element 22, and a coaxial connector 28 of the coaxial line path is connected to the GND 23 of the patch antenna 20. There is an antenna element disclosed in JP-A-2004-260786 (laid-open on Sep. 16, 2004) as one example of the antenna having such a structure.
In the power supply method, a method for supplying power by arranging a matching portion 25 as shown in FIG. 12 is also generally well utilized as well as the offset power supply (see Japanese patent No. 3,273,402 (registered on Feb. 1, 2002)). However, in each of these methods, a patch antenna 20′ using the general dielectric substrate 21 has a high Q-value and a narrow frequency band. For example, if the frequency band is a 1 GHz band, it is very difficult to ensure that VSWR (Voltage Standing Wave Ratio) is 1.5 or less and the frequency band is 10 MHz or more even if a parasitic element is formed when a glass epoxy substrate of t=1.6 mm in thickness is used as the dielectric substrate 21.
Therefore, there is also a method for forming the patch antenna through a layer having a dielectric constant of 1, i.e., the air to widen the frequency band. FIGS. 13A and 13B show the structure of a patch antenna 20″ in which the air layer is trapped. FIG. 13A is a plan view of the patch antenna 20″. FIG. 13B is a cross-sectional view seen from line B-B′ of the patch antenna 20″ shown in FIG. 13A. The patch antenna 20″ is structurally the same as the patch antenna 20 of FIGS. 11A and 11B using the dielectric substrate 21. However, an air gap area G is arranged between the radiating element 22 and the GND 23 to secure a wide frequency band. A spacer 26 is arranged in the air gap area G, and maintains the distance between the radiating element 22 and the GND 23. Specifically, the patch antenna 20″ of FIGS. 13A and 13B secures a wide frequency band by widely designing the width of the patch antenna having only the thickness of the dielectric substrate 21 in the patch antenna 20 of FIGS. 11A and 11B.
However, in the patch antenna 20 of FIGS. 11A and 11B, the coaxial connector 8 is arranged on the rear face of the patch antenna 20. Therefore, when the patch antenna 20 is arranged in a wall, etc., the coaxial connector 8 becomes an obstacle. Specifically, there is a limit to the degree of freedom of the configuration. Further, in the patch antenna 20″ of FIGS. 13A and 13B, as mentioned above, a wide frequency band can be also secured by arranging the air gap area G between the radiating element 22 and the GND 23, but the coaxial connector 28 is obliged to be arranged on the rear face of the GND 23 as shown in FIGS. 13A and 13B when the offset power supply is performed. Accordingly, similar to the patch antenna 20 of FIGS. 11A and 11B, there is a limit in the degree of freedom of the configuration.
With respect to the degree of freedom of the configuration, a patch antenna 20′″ having another structure is shown in FIGS. 14A and 14B. FIG. 14A is a plan view of the patch antenna 20′″. FIG. 14B is a cross-sectional view seen from line B-B′ of the patch antenna 20″′ shown in FIG. 14A. As shown in FIG. 14B, this patch antenna 20′″ is set to a structure in which no coaxial connector 28 is arranged on the rear face of the GND 23. Thus, the above restriction of the arrangement is not removed.
However, in the patch antenna 20″ of FIGS. 13A and 13B and the patch antenna 20′″ of FIGS. 14A and 14B, the wide frequency band can be secured, but a problem exists in that the central conductor 27 of the coaxial line path is in a very unstable state.
For example, in both the patch antenna 20″ shown in FIGS. 13A and 13B and the patch antenna 20′″ shown in FIGS. 14A and 14B, the distance between the radiating element 22 and the GND 23 is widely set. Therefore, the wide frequency band can be secured, but the central conductor 27 of the coaxial line path attains a very unstable state without supporting this central conductor 27 by another member within this air gap area G. In such a state, the central conductor 27 is easily deteriorated in characteristics by an impact from the exterior, a vibration at a manufacturing time, etc.
Further, it is necessary to closely arrange the central conductor 27 on the coaxial line path between the radiating element 22 and the GND 23. Therefore, a problem exists in that assembly work property is very poor. Further, in the patch antenna 20″ of FIGS. 13A and 13B, there is a case in which the central conductor 27 of the coaxial line path is connected to the radiating element 22 in a curved state during manufacturing. In this case, a problem exists in that an individual difference of the antenna characteristics is caused in accordance with a degree of curvature of the central conductor 27.
With respect to the assembly work property, there is a patch antenna 20″″ of the structure shown in FIGS. 15A and 15B. FIG. 15A is a plan view of the patch antenna 20″″. FIG. 15B is a cross-sectional view seen from line B-B′ of the patch antenna 20″″ shown in FIG. 15A. As shown in FIG. 15A, the connecting work property of the central conductor 27 of the coaxial line path to the radiating element 22 can be raised by arranging a matching portion 25.
However, in the patch antenna 20″″ of FIGS. 15A and 15B, a problem exists in that the size of the patch antenna 20″″ itself is increased. Specifically, in the patch antenna 20″″ of FIGS. 15A and 15B, a power supply system for arranging the matching portion 25 is used instead of the offset power supply system. Therefore, in comparison with the case adopting the offset power supply system, the size of the patch antenna is increased by an area (length) corresponding to the matching portion 25.